Anode modification for organic light emitting diodes

ABSTRACT

An organic light emitting device is provided which comprises a cathode ( 51 ), an anode ( 48, 47, 46 ), and an organic region ( 50, 49 ) in which electroluminescence takes place if a voltage is applied between said anode and cathode The anode comprises a metal layer ( 46 ), a barrier layer ( 47 ), and an anode modification layer ( 48 ) Light is emitted through the cathode ( 51 ).

TECHNICAL FIELD

[0001] The present invention pertains to organic electroluminescentdisplays and methods for making the same.

BACKGROUND OF THE INVENTION

[0002] Organic electroluminescence (EL) has been studied extensivelybecause of its possible applications in discrete light emitting diodes(LED), arrays and displays. Organic materials can potentially replacesemiconductors in many LED applications and enable wholly newapplications. The ease of organic LED (OLED) fabrication and thecontinuing development of improved organic materials promise novel andinexpensive OLED display possibilities.

[0003] Organic EL at low efficiency was reported many years ago in e.g.Helfrich et al., Physical Review Letters, Vol. 14, No. 7, 1965, pp.229-231. Recent developments have been spurred by two reports ofefficient organic EL: C. W. Tang et al., Applied Physics Letters, Vol.51, No. 12, 1987, pp.913-915, and Burroughes et al., Nature, Vol. 347,1990, pp. 539. Tang used vacuum deposition of molecular compounds toform OLEDs with two organic layers. Burroughes spin coated a polymer,poly(p-phenylenevinylene), to form a single-organic-layer OLED. Theadvances described by Tang and in subsequent work by N. Greenham et al.,Nature, Vol. 365, 1993, pp. 628-630, were achieved mainly throughimprovements in OLED design derived from the selection of appropriateorganic rtiultilayers and electrode metals.

[0004] The simplest possible OLED structure, depicted in FIG. 1A,consists of an organic emission layer 10 sandwiched between cathode 11and anode 12 electrodes which inject electrons (e⁻) and holes (h⁺),respectively, which meet in the emission layer 10 and recombineproducing light. It has been shown (D. D. C. Bradley, Synthetic Metals,Vol. 54, 1993, pp. 401-405, J. Peng et al., Japanese Journal of AppliedPhysics, Vol. 35, No. 3A, 1996, pp. L317-L319, and I. D. Parker, Journalof Applied Physics, Vol. 75, No. 3, 1994, pp. 1656-1666) that improvedperformance is achieved if the electrode work functions match therespective molecular orbitals (MO) of the organic layer 10. Such animproved structure is shown in FIG. 1B where optimized electrodematerials 13 and 14 reduce the energy barriers to carrier injection intothe organic layer 10. Still, single organic layer structures performpoorly because electrons can traverse the organic layer 10 reaching theanode 14, or holes may reach the cathode 13, in either case resulting incurrent without light, and lower OLED efficiency.

[0005] Balanced charge injection is also important. For example, anexcellent anode is of limited use if the cathode has a large energybarrier to electron injection. FIG. 2 illustrates a device with a largeelectron barrier 16 such that few electrons are injected, leaving theholes no option but to recombine non-radiatively in or near the cathode15. The anode and cathode materials should be evenly matched to theirrespective MOs to provide balanced charge injection and optimized OLEDefficiency.

[0006] An improved structure in which the electron and hole transportfunctions are divided between separate organic layers, an electrontransport layer 20 (ETL) and a hole transport layer (HTL) 21, is shownin FIG. 3. In C. W. Tang et al., Journal of Applied Physics, Vol. 65,No. 9, 1989; pp. 3610-3616, it is described that higher carrier mobilityis achieved in a two organic layer OLED design, and this led to reducedOLED series resistance enabling equal light output at lower operatingvoltage. The electrodes 22, 23 can be chosen individually to match tothe ETL 20 and HTL 21 MOs, respectively, while recombination occurs atthe interface 24 of organic layers 20 and 21, far from either electrode.As electrodes, Tang used a MgAg alloy cathode and transparentIndium-Tin-Oxide (ITO) deposited on a glass substrate as the anode.Egusa et al. in Japanese Journal of Applied Physics, Vol. 33, No. 5A,1994, pp. 2741-2745 have shown that proper selection of the organicmultilayer materials leads to energy barriers blocking both electronsand holes at the organic interface. This is illustrated in FIG. 3 inwhich electrons are blocked from entering the HTL 21 and holes fromentering the ETL 20 by a clever choice of organic materials. Thisfeature reduces quenching near the contacts (as illustrated in FIG. 2)and also promotes a high density of electrons and holes in the smallinterface volume providing enhanced radiative recombination.

[0007] With multilayer device architectures now well understood andwidely used, a remaining performance limitation of OLEDs is theelectrodes. The main figure of merit for electrode materials is theposition of the electrode Fermi energy relative to the relevant organicMO. In some applications it is also desirable for an electrode to beeither transparent or highly reflective to assist light extraction.Electrodes must also be chemically inert with respect to the adjacentorganic material to provide long term OLED stability.

[0008] Much attention has been paid to the cathode, largely because goodelectron injectors are low work function metals which are alsochemically reactive and oxidize quickly in atmosphere, limiting the OLEDreliability and lifetime. Much less attention has been paid to theoptimization of the anode contact, since conventional ITO anodesgenerally outperform the cathode contact leading to an excess of holes.Due to this excess, and the convenience associated with the transparencyof ITO, improved anodes have not been as actively sought as improvedcathodes.

[0009] ITO is by no means an ideal anode, however. ITO is responsiblefor device degradation as a result of In diffusion into the OLEDeventually causing short circuits as identified by G. Sauer et al.,Fresenius J. Anal. Chem., pp. 642-646, Vol. 353 (l,995). ITO ispolycrystalline and its abundance of grain boundaries provides amplepathways for contaminant diffusion into the OLED. Finally, ITO is areservoir of oxygen which is known to have a detrimental effect on manyorganic materials (see J. C. Scott, J. H. Kaufman, P. J. Brock, R.DiPietro, 3. Salem, and J. A. Goita, J. Appl. Phys., Vol. 79, p. 2745,1996). Despite all of these problems, ITO anodes are favored because nobetter transparent electrode material is known and ITO provides adequatestability for many applications.

[0010] While conventional OLEDs extract light through the ITO anode,architectures relying on light extraction through a highly transparentcathode (TC) are desirable for transparent OLEDs or OLEDs fabricated onan opaque substrate. Si is an especially desirable OLED substratebecause circuits fabricated in the Si wafer can be cheaply integratedwith drive circuitry providing display functions. Given theminaturization and outstanding performance of Si circuitry, a highinformation content OLED/Si display could be inexpensively fabricated ona Si integrated circuit (IC).

[0011] The simplest approach incorporating a TC is to deposit a thin,semi-transparent low work function metal layer, e.g. Ca or MgAg,followed by ITO or another transparent, conducting material ormaterials, e.g. as reported in Bulovic et al., Nature, Vol. 380, No. 10,1996 p. 29, or in the co-pending PCT patent application PCT/IB96/00557,published on Dec. 11, 1997 (publication number WO97147050). To maximizethe efficiency of such a TC OLED, a highly reflective anode which candirect more light out through the TC is desired. Consequently, the lowreflectivity of ITO is a disadvantage in TC OLEDs.

[0012] Alternatively, for some applications it may be more important toincrease the contrast ratio of the OLEDs or display based thereon. Inthis case, a TC OLED could benefit from a non-reflective, highlyabsorbing anode. Again the optical characteristics of ITO are adisadvantage.

[0013] High work function metals could form highly reflective anodes forTC OLEDs. Some of these metals, e.g. Au, have a larger work functionthan ITO (5.2 eV vs. 4.7 eV), but lifetime may be compromised because ofhigh diffusivity in organic materials. Like In from ITO, only worse, Audiffuses easily through many organic materials and can eventually shortcircuit the device.

[0014] Efforts have been made to fabricate OLEDs on Si substrates(Parker and Kim, Applied Physics Letters, Vol. 64, No. 14, 1994, pp.1774-1776). Si, due to its small bandgap and moderate work function, hasa large barrier for both electron and hole injection into organic MOs,and therefore performs poorly as an electrode. Parker and Kim improvedthe situation somewhat by adding a SiO₂ interlayer between the Si andOLED. A voltage drop across the SiO₂ insulator permitted the Si bands toline up with their organic MO counterpart and electrons or holes fromthe Si to tunnel through the SiO₂ into the organic MO. However, therequired voltage drop across the SiO₂ raised the OLED turn-onvoltage >10 V, malting these OLEDs inefficient. Low voltage OLED/Sidesigns are desirable not just to improve efficiency, but also tofacilitate circuit design since sub-micron Si transistors cannot easilyproduce drive voltages >10 V. For anodes, more desirable than atunneling insulator surface modification like SiO₂ is one which raisesthe Si surface work function thereby lowering the OLED operatingvoltage.

[0015] As can be seen from the above examples and description of thestate of the art, electrode materials must be improved to realize OLEDs,and displays based thereon, with superior reliability and efficiency,and to enable novel architectures, such as devices emitting through aTC. In particular, to fabricate an OLED array or display on a Sisubstrate, an improved anode compatible with Si IC technology isrequired for optimized TC OLED architectures.

[0016] It is an object of the present invention to provide new andimproved organic EL devices, arrays and displays based thereon.

[0017] It is a further purpose of the present invention to provide newand improved organic EL devices, arrays and displays based thereonoptimized for eight emission through a transparent cathode electrodewith improved efficiency, lower operating voltage, or steepercurrent/voltage characteristic and increased reliability, stability andlifetime.

[0018] It is another object of the present invention to provide new andimproved anodes for organic EL devices, arrays and displays fabricatedon Si substrates.

[0019] It is a further object to provide a method for making the presentnew and improved organic EL devices, arrays and displays.

SUMMARY OF THE INVENTION

[0020] The above objects have been accomplished by providing an OLEDhaving a cathode, an anode, and an organic region sandwiched in between,said anode being composed of

[0021] a metal layer,

[0022] an anode modification layer, and

[0023] at least one barrier layer,

[0024] said anode being arranged such that said anode modification layeris in contact with said organic region and light is extracted throughsaid cathode.

[0025] Any kind of metal is suited as metal layer in connection with thepresent invention. Examples are Al, Cu, Mo, Ti, Pt, Ir, Ni, Au, Ag, andany alloy thereof, or any metal stack such as Pt on Al and the like.

[0026] The inventive approach is specifically designed for thefabrication of OLEDs on top of Si, preferably Si crystalline wafersincorporating pre-processed integrated display circuitry (hereinreferred to as Si IC). The present invention is designed to modify theexisting Si device metallization into a stable OLED anode having goodhole injection properties. For OLEDs on top of a Si IC, the metal layerin the present invention is generally the final metallization layer ofthe Si IC process, which consistent with present Si technology isnormally Al, Cu or an alloy thereof. Neither Al, Cu or Al:Cu alloysperform well as OLED anodes, but they do provide excellent visiblespectrum reflectivity which increases the amount of light extractedthrough a TC. The Si IC metallization surface can vary widely in termsof oxide thickness, roughness and surface contaminants depending onnumerous factors, including the fabrication process, the time between ICfabrication and OLED deposition, and the environment in which the Si ICwas stored and shipped. For reproduceable fabrication of efficient OLEDsthe Si metallization anode properties must be improved and effectsarising from variations of the initial state of the metal surface mustbe eliminated.

[0027] The inventive approach is also suited for use with pixel anddrive circuitry comprising polysilicon or amorphous silicon devices.

[0028] The anode modification layer in the present invention is mainlyselected for its high work function which provides efficient holeinjection into OLEDs. The anode modification layer must form a stableinterface with the adjacent organic layer being part of the so-calledorganic region (e.g. the organic HTL) to insure consistent OLEDperformance over an extended time period. The anode modification layercan be conductive or insulating, but it should be sufficiently thin thatit contributes negligibly both to the OLED series resistance and opticalabsorption losses. Oxides are well suited as anode modification layers.The thickness of the anode modification layer is preferably between 0.5nm and 10 nm.

[0029] The barrier layer or layers in the present invention isolates theanode modification layer, from the metal layer by forming a physical andchemical barrier, while permitting charge to pass thIS freely throughits interfaces with the metal layer and anode modification layer. Thebarrier layer(s) provides a consistent and reproduceable surface for thedeposition or formation of the anode modification layer regardless ofthe metal layer composition or initial state of its surface. The barrierlayer(s) can be conductive or insulating, but it (they) should besufficiently thin that it (they) contributes negligibly to the OLEDseries resistance. Alternatively, the barrier layer(s) can be highlyreflective which avoids absorption losses. The thickness of the barrierlayer is preferably between 5 nm-100 nm. Well suited are barrier layerscomprising TiN or TiNC, for example.

[0030] For formation on a Si wafer, all of the layers comprising theanode must be depositable or formable onto the wafer using processessufficiently gentle that underlying Si circuitry is undamaged, i.e. atlow temperature causing little chemical or physical damage.

[0031] In one embodiment of the present invention, a single ormultilayer OLED structure having a TC fabricated on a Si substrateincorporates a multilayer anode structure comprising a metal layer, ananode modification layer, and an intermediate barrier layer(s), suchthat the anode is stable and efficient at hole injection.

[0032] The introduction of such an anode into the OLED structure leadsto the following advantages:

[0033] Low voltage hole injection via both the high work function of theanode modification layer and the free passage of charge from the metallayer, through the barrier layer(s) into the anode modification layer.

[0034] Stable OLED operation over an extended time period via thechemical and physical barrier the barrier layer(s) provides between themetal layer and anode modification layer, and the stability of the anodemodification layer interface with the adjacent organic HTL.

[0035] Efficient light extraction through the TC aided by the highreflectivity and low absorption of the metal layer, barrier layer(s) andanode modification layer stack.

DESCRIPTION OF THE DRAWINGS

[0036] The invention is described in detail below with reference to thefollowing schematic drawings. It is to be noted that the Figures are notdrawn to scale.

[0037]FIG. 1A shows the band structure of a known OLED having anemission layer and two electrodes (Prior Art).

[0038]FIG. 1B shows the band structure of another known OLED having anemission layer and two metal electrodes, with work functions chosen suchthat the energy barrier for carrier injection is reduced (Prior Art).

[0039]FIG. 2 shows the band structure of another known OLED having anemission layer and two metal electrodes, the work function of the anodebeing chosen such that the energy barrier for hole injection is low,whereas the work function of the cathode poorly matches the emissionlayer yielding little electron injection and little radiativerecombination in said emission layer (Prior Art).

[0040]FIG. 3 shows the band structure of another known OLED having anelectron transport layer and hole transport layer (Prior Art).

[0041]FIG. 4 is a schematic cross section of a first embodiment of thepresent invention.

[0042]FIG. 5 is a schematic cross section of a second embodiment of thepresent invention.

[0043]FIG. 6 is a schematic cross section of a third embodiment of thepresent invention.

DESCRIPTION OF PREFERRED EMBODIMENTS:

[0044] A first improved structure benefiting from the inventive anodeapproach is illustrated and discussed in connection with FIG. 4.

[0045] To provide high performance TC OLED devices fabricated on Sisubstrates, improved structures benefiting from the inventive anodeapproach, as illustrated in FIGS. 4-6, are provided, enabling new arrayand display applications. Three embodiments of improved OLEDsincorporating the inventive anodes are detailed in connection with FIGS.4-6.

[0046] A first embodiment is depicted in FIG. 4. A TC OLED is shownwhich is formed on a substrate 45. Since in the present configurationthe electroluminescent light 52 is emitted through the top electrode(cathode 51), almost any kind of substrate 45 can be used. Examples areSi, glass, quartz, stainless steel, and various plastics. The inventiveanode, comprising a metal layer 46, a barrier layer 47, and an anodemodification layer 48 is situated on said substrate 45. Any kind ofmetal is suited as metal layer 46. Examples are Al, Cu, Mo, Ti, Pt, Ir,Ni, Au, Ag, and any alloy thereof, or any metal stack such as Pt on Aland the like. Depending on the embodiment, particularly well suited aremetals which provide visible spectrum reflectivity.

[0047] The barrier layer 47 isolates the anode modification layer 48from the metal layer 46 by forming a physical and chemical barrier,while permitting charge to pass freely through its interfaces with themetal layer 46 and anode modification layer 48. Note that the inventiveanode might comprise one or more barrier layers. The barrier layer(s) 47provides a consistent and reproduceable surface for the formation ordeposition of the anode modification layer 48 regardless of the metallayer composition or initial state of its surface. The barrier layer(s)47 can be conductive or insulating, but it (they) should be sufficientlythin that it (they) contributes negligibly to the OLED seriesresistance. Alternatively, the barrier layer(s) 47 can be reflectivewhich avoids absorption losses. The thickness of the barrier layer ispreferably between 5 nm-100 nm.

[0048] The anode modification layer 48 is mainly selected for its highwork function which provides efficient hole injection into the organicregion of the OLED. The anode modification layer 48 must form a stableinterface with the adjacent organic emission layer (EML) 49 to insureconsistent OLED performance over an extended time period. The anodemodification layer 48 can be conductive or insulating, but it must besufficiently thin that it contributes negligibly to both the OLED seriesresistance and the optical absorption losses. The thickness of the anodemodification layer is preferably between 0.5 nm and 10 nm.

[0049] A TC 51 is situated on the EML 49. The electroluminescence takesplace within the EML 49. As indicated in FIG. 4, part of the light isemitted directly through the EML 49 and the TC 51 into the half spaceabove the OLED. Another part of the light travels towards the inventiveanode structure. The anode structure reflects the light such that it isalso emitted into the half space above the OLED. TABLE 1 Exemplarydetails of the first embodiment present Layer No. Material Thicknessexample Substrate 45 Quartz  0.05-5 mm 800 μm Metal Layer 46 Ti/Al  0.01-0.7 μm/  2 nm 0.05-3 μm Barrier Layer 47 Ni 0.01-2 μm  30 nmAnode modification 48 ITO 0.003-2 μm   7 nm Layer Emission Layer 49 PPV   50-500 nm 200 nm Transparent Cathode 51 Li: Al alloy    50-1000 nm120 nm

[0050] A second embodiment of the inventive anode for TC OLEDsfabricated on Si substrates is depicted in FIG. 5. From the substrate 25up, listed in the order of deposition, is a Si IC/InN/InNO,/HTU/ETL/ICOLED structure. The Si IC 25 in FIG. 5 comprises an Al top metal contactpad 26 which also serves as the metal layer in the inventive anode.After completion of the Si IC fabrication, an InN barrier layer 27 isdeposited directly onto the substrate such that it overlaps at least thecontact pad 26. The sample is then oxidized in an oxygen plasma, orequivalently in an air, steam, ozone or other oxidizing environment toprepare an INNOX surface anode modification layer 28 capable of lowvoltage hole injection into the organic HTL 29. Electrons injected intothe ETL 30 from the TC 31 recombine with the holes in the organic regionproducing EL 32 which is extracted through the TC 31. The organic regionin the present embodiment comprises an HTL 29 and an ETL 30. It is to benoted that the organic region in any case at least comprises one organiclayer (see first embodiment, for example). The Si IC 25 might compriseintegrated circuitry which is not illustrated in FIG. 5, for sake ofsimplicity. Instead of InN other group III nitrides might be used, forexample.

[0051] InN is an excellent barrier layer material because it is adegenerate semiconductor which has both excellent transparency and isconductive, but not so conductive that lateral conduction through theInN barrier layer 27 between adjacent Al metal pads 26 on the Si IC 25causes electrical crosstalk. InN having these properties can bedeposited at or near room temperature as described in Beierlein et al.,Materials Research Society Internet Journal of Nitride SemiconductorResearch, Vol. 2, Paper 29. InN is also a convenient choice because itssurface work function can be increased by oxidation thus forming anInNOX anode modification layer 28 directly on the InN barrier layer.Equivalently, the InNO, anode modification layer 28 could be directlydeposited onto the InN.

[0052] Several methods of deposition of oxide-based anode modificationlayers are listed below:

[0053] chemical vapor deposition (CVD), including plasmaenhancedchemical vapor deposition (PECVD);

[0054] sputter deposition or reactive (e.g. in an oxygen environment)sputter deposition;

[0055] thermal evaporation;

[0056] electron-beam evaporation;

[0057] oxygen plasma (plasma-assisted oxidation);

[0058] thermal annealing in an oxidizing environment;

[0059] UV-ozone treatment;

[0060] wet-chemical oxidation;

[0061] electrochemical oxidation;

[0062] spin-coating from solution.

[0063] A completely different anode modification layer 28 could also besubstituted, e.g. ITO, ZnO, MgO, Sn₂O₃, In₂O₃, RuO₂, or V₂O₅, or similaroxides, just to give some examples. Similarly, a completely differentmetal layer 26 could also be substituted. The degeneracy of the InNsemiconductor insures that charge can pass easily from the metal layer26 into the InN barrier layer 27, regardless of the composition orinitial state of the metal layer-26 surface. For the same reason, chargecan also traverse the InN barrier layer 27 and into the anodemodification layer 28 without significant series resistance. Thestrength of the highly polar In-N bond, insures that InN 27 acts as anexcellent chemical and physical barrier to corrosion or diffusionbetween the metal layer 26 and the anode modification layer 28.

[0064] The device depicted in FIG. 5 benefits from the high reflectivityof the Al metal layer 26, and the low visible spectrum absorption of theInN barrier layer 27 and the InNO, anode modification layer 28, whichpermits much of the EL 32 emitted towards the substrate to be reflectedback through the TC 31.

[0065] Devices having the anode structure depicted in FIG. 5 exhibithigher quantum and power efficiencies than comparable structures havingconventional ITO anodes as a result of more balanced charge injectionbetween the Al/InN/InNO_(x) anode and the TC. TABLE 2 Exemplary detailsof the second embodiment present Layer No. Material Thickness exampleSubstrate 25 Si IC  0.05-1 mm 500 μm Metal Layer 26 Aluminum 0.05-2 μm500 nm Barrier Layer 27 InN 0.01-1 μm  20 nm Anode modification 28InNO_(x) 0.001-1 μm   5 nm Layer Hole Transport 29 TAD    10-200 nm  50nm Layer Electron Transport 30 Alq₃    10-200 nm  50 nm LayerTransparent Cathode 31 MgAg/ITO     1-20 nm/  80 nm/    10-1000 nm  50nm

[0066] A third embodiment of the inventive anode for TC OLEDs fabricatedon Si substrates is depicted in FIG. 6. From the substrate 33 up, listedin the order of deposition, is a Si IC/Ni/NiO_(x)/V₂O₅/HTL/EML/ETL/TCOLED structure. The Si IC 33 in FIG. 6 comprises an Al:Cu alloy topmetal contact 34 which serves as the metal layer in the inventive anode.After completion of the Si IC, a two layer Ni 35/NiO_(x) 36 barrierlayer is deposited in sequence directly onto those parts of thesubstrate 33 which are covered by the metal contact layer 34, oralternatively, the Ni 35 barrier layer is deposited, and its surface issubsequently oxidized to form the NiO_(x) 36 barrier layer. Theinventive anode is completed by the deposition of the V₂O₅ anodemodification layer 37 capable of injecting holes into the HTL 38.Electrons injected into the ETL 40 from the TC 41 recombine in theorganic emission layer 39 producing EL 42 which is extracted through theTC 41. The circuitry in the Si substrate is not shown for simplicity.

[0067] We note that an arbitrarily large number of barrier layers 35, 36can be inserted between the metal layer 34 and the anode modificationlayer 37 of the inventive anode provided that they do not reduce deviceefficiency through excessive series resistance at their interfaces or intheir bulk. The Ni 35/NiO_(x) 36 barrier layer structure of the presentembodiment relies on the high reflectivity of the Ni 35 metal and thetransparency and thinness of the insulating NiO_(x) 36 layer to insuregood device efficiency.

[0068] We note that the third embodiment without the Ni and NiO, layersis unstable due to a chemical reaction between the Al:Cu alloy 34 andthe anode modification layer 37. The oxidation of the Ni 35 surface, oralternatively the deposition of an additional barrier layer furtherchemically isolates the Ni metal from the anode modification layer 37.Because the Ni 35 barrier layer is highly conductive, it must be verythin, or patterned (as shown in FIG. 6) to avoid lateral conductionbetween adjacent IC metallizations 34 (not shown in FIG. 6).

[0069] Devices having the inventive metal/Ni/NiO_(x)/V₂O₅ anodestructure depicted in FIG. 6 exhibit steeper current/voltagecharacteristics than conventional ITO anodes and similar powerefficiencies. TABLE 3 Exemplary details of the third embodiment presentLayer No. Material Thickness example Substrate 33 Si IC  0.05- 500 μm 5mm Metal Layer 34 Al—Cu  0.05- 500 nm 5 μm Barrier Layer 35 Ni 0.1-  2.5nm 1000 nm  Barrier Layer 36 NiO_(x) 0.1-  1 nm 10 nm  Anodemodification 37 V₂O₅ 0-1 μm   5 nm layer Hole Transport Layer 38 NPB10-200 nm   50 nm Emission Layer 39 Alq₃:Rubrene 1-100 nm  15 nmElectron Transport 40 Alq₃ 10-200 nm   50 nm Layer Transparent Cathode41 Ca/InGaN/ITO 1-20 nm/  10 nm/ 10-100 nm/  80 nm/ 10-1000 nm  150 nm

[0070] In the following, some display embodiments, based on and enabledby the present invention, are addressed.

[0071] Any of the three embodiments, or modifications thereof, can bepart of a display or array. The Si substrate, for example, mightcomprise integrated circuitry which drives the pixels of the OLEDsformed thereon. For this purpose, the inventive anode might be connectedto the metal contact of an active matrix element formed in the Si ICsubstrate. If the circuitry is patterned appropriately, individualpixels or groups of pixels can be turned on and off.

[0072] Arrays and displays can be realized with high quantum and powerefficiencies, lower threshold voltages, and/or steep current/voltagecharacteristics. The inventive anode is compatible with many knownapproaches.

[0073] It is advantageous to integrate OLEDs onto Si substrates becauseprior to OLED deposition, the substrate can be fabricated to contain theactive Si devices, such as for example an active matrix, drivers, memoryand so forth. Such an OLED on Si structure can be a very inexpensivesmall area organic display with high resolution and performance. An OLEDor OLED array may either be grown directly on such a Si substratecarrying Si devices, or it may be fabricated separately and assembled ina flip-chip fashion onto the Si circuitry later.

[0074] One problem is that the Si metallization is typically Al or an Alalloy which are poor OLED anode or cathode materials. The inventiveanode permits a stable, low voltage hole contact to be formed on top ofthe standard Si process metallizations.

[0075] In the following some examples of the different organic materialswhich can be used are given.

Electron transport/Emitting materials:

[0076] Alq₃, Gaq₃, Inq₃, Scq₃, (q refers to 8-hydroxyquinolate or it'sderivatives) and other 8-hydroxyquinoline metal complexes such as Znq₂,Beq₂, Mgq₂, ZnMq₂, BeMq₂, BAlq, and AlPrq₃, for example. These materialscan be used as the ETL or emission layer.

[0077] Other classes of electron transporting materials areelectron-deficient nitrogen-containing systems, for example oxadiazoleslike PBD (and many derivatives), and triazoles, for example TAZ(1,2,4-triazole).

[0078] These functional groups can also be incorporated in polymers,starburst and spiro compounds. Further classes are materials containingpyridine, pyrirnidine, pyrazine and pyridazine functionalities.

[0079] Finally, materials containing quinoline, quinoxaline, cinnoline,phthalazine and quinaziline functionalities are well known for theirelectron transport capabilities.

[0080] Other materials are didecyl sexitniophene (DPS6T),bis-triisopropylsilyl sexitniophene (2D6T), azomethin-zinc complexes,pyrazine (e.g. BNVP), styrylanthracene derivatives (e.g. BSA- 1, BSA-2),non-planar distyrylarylene derivatives, for example DPVBi (see C.Hosokawa and T. Kusumoto, International Symposium on Inorganic andOrganic Electroluminescence 1994, Hamamatsu, 42), cyano-substitutedpolymers such as cyano-PPV (PPV means poly(p-phenylenevinylene)) andcyano-PPV derivatives.

[0081] The following materials are particularly well suited as

Emission Layers and Dopants

[0082] Anthracene, pyridine derivatives (e.g. ATP), Azomethin-zinccomplexes, pyrazine (e.g. BNVP), styrylanthracene derivatives (e.g.BSA-1, BSA-2), Coronene, Coumarin, DCM compounds (DCM1, DCM2), distyrylarylene derivatives (DSA), alkyl-substituted distyrylbenzene derivatives(DSB), benzimidazole derivatives (e.g. NBI), naphthostyrylamninederivatives (e.g. NSD), oxadiazole derivatives (e.g. OXD, OXD-1, OXD-7),N,N,N′,N′-tetrakis(m-methylphenyl)-1,3-diaminobenzene (PDA), peryleneand perylene derivatives, phenyl-substituted cyclopentadienederivative's, 12-phthaloperinone sexithiophene (6T), polythiophenes,quinacridones (QA) (see T. Wakimoto et al., International Symposium onInorganic and Organic Electroluminescence, 1994, Harnamatsu, 77), andsubstituted quinacridones (MQA), rubrene, DCJT (see for example: C.Tang, SID Conference San Diego; Proceedings, 1996, 181), conjugated andnon-conjugated polymers, for example PPV and PPV derivatives, dialkoxyand dialkyl PPV derivatives, for example

[0083] MEH-PPV(poly(2-methoxy)-5-(2′-ethylhexoxy)-1,4-phenylene-vinylene),poly(2,4-bis(cholestanoxyl)- 1,4-phenylene-vinylene) (BCHA-PPV), andsegmented PPVs (see for example: E. Staring in International Symposiumon Inorganic and Organic Electroluminescence, 1994, Hamamatsu, 48, andT. Oshino et al. in Sumitomo Chemicals, 1995 monthly report).

Hole Transport Layers and Hole Injection Layers

[0084] The following materials are suited as hole injection layers andhole transport layers. Materials containing aromatic amino groups, liketetraphenyldiarinodiphenyl (TPD- 1, TPD-2, or TAD) and NPB (see C. Tang,SID Meeting San Diego, 1996, and C. Adachi et al. Applied PhysicsLetters, Vol. 66, p. 2679, 1995), TPA, NIPC, TPM, DEH (for theabbreviations see for example: P. Borsenberger and D. S. Weiss, OrganicPhotoreceptors for Imaging Systems, Marcel Dekker, 1993). These aromaticamino groups can also be incorporated in polymers, starburst (forexample: TCTA, m-MTDATA, see Y. Kuwabara et al., Advanced Materials, 6,p. 677, 1994, Y. Shirota et al., Applied Physics Letters, Vol. 65, p.807, 1994) and Spiro compounds.

[0085] Further examples are: Copper(II) phthalocyanine (CuPc), A(N,N′-diphenyl-N,N′-bis-(4-phenylphenyl)-1,1′-biphenyl-4,4′-diamine),distyryl arylene derivatives (DSA), naphthalene, naphthostyrylarninederivatives (e.g. NSD), quinacridone (QA), poly(3-methylthiophene)(P3MT) and its derivatives, perylene and perylene derivatives,polythiophene (PT), 3,4,9,10-perylenetetracarboxylic dianhydride(PTCDA), PPV and some PPV derivatives, for example MEH-PPV,poly(9-vinylcarbazole) (PVK), discotic liquid crystal materials (HPT).

[0086] There are many other organic materials known as being good lightemitters, charge transport materials, and charge injection materials,and many more will be discovered. These materials can be used as wellfor making light emitting structures according to the present invention.More information on organic materials can be found in text books andother well known publications, such as the book “Inorganic and OrganicElectroluminescence”, edited by R. H. Mauch et al., Wissenschaft undTechnik Verlag, Berlin, Germany, 1996, and the book “1996 SIDInternational Symposium, Digest of Technical Papers”, first edition,Vol. XXVII, May 1996, published by Society for Information Display, 1526Brookhollow Dr., Suite 82, Santa Ana, Calif., USA.

[0087] Additionally, blend (i.e. guest-host) systems containing activegroups in a polymeric binder are also possible. The concepts employed inthe design of organic materials for OLED applications are to a largeextent derived from the extensive existing experience in organicphotoreceptors. A brief overview of some organic materials used in thefabrication of organic photoreceptors is found in the above mentionedpublication of P. Brosenberger and D. S. Weiss, and in Teltech,Technology Dossier Service, Organic Electroluminescence (1995), as wellas in the primary literature.

[0088] OLEDs have been demonstrated using polymeric, oligomeric andsmall organic molecules. The devices formed from each type of moleculeare similar in function, although the deposition of the layers varieswidely. The present invention is equally valid in all forms describedabove for organic light emitting devices based on polymers, oligomers,or small molecules.

[0089] Small molecule devices are routinely made by vacuum evaporation.This is compatible with the process used for the formation of theinventive anode.

[0090] Evaporation can be performed in a Bell jar type chamber withindependently controlled resistive and electron-beam heating of sources.It can also be performed in a molecular beam deposition systemincorporating multiple effusion cells and sputter sources. Oligomericand polymeric organics can also be deposited by evaporation of theirmonomeric components with later polymerization via heating or plasmaexcitation at the substrate. It is therefore possible to copolymerize orcreate mixtures by co-evaporation.

[0091] In general, polymer containing devices (single layer, multilayer,polymer blend systems, etc.) are made by dissolving the polymer in asolvent and spreading it over the substrate either by spin coating orthe doctor blade technique. After coating the substrate, the solvent isremoved by evaporation or otherwise. This method allows the fabricationof well defined multilayer organic stacks, provided that the respectivesolvents for each subsequent layer do not dissolve previously depositedlayers.

[0092] Additionally, hybrid devices containing both polymeric andevaporated small organic molecules are possible. In this case, thepolymer film is generally deposited first, since evaporated smallmolecule layers often cannot withstand much solvent processing.

1. An organic electroluminescent device comprising an anode, a cathode,and an organic region being sandwiched in between, characterized in thatthe anode comprises a metal layer, a barrier layer, and an anodemodification layer, wherein said anode is arranged such that said anodemodification layer is in contact with said organic stack and light isemitted through said cathode.
 2. The device of claim 1, wherein thesurface of the barrier layer is oxidized to form the anode modificationlayer.
 3. The device of claim 1, wherein the anode modification layer isdeposited onto the barrier layer.
 4. The device of claim 1, wherein thebarrier layer comprises a group m nitride, such as InN.
 5. The device ofclaim 1, wherein the metal layer comprises Al or Cu.
 6. The device ofclaim 1, wherein the anode modification layer has a high workfunction.7. The device of claim 1, wherein the metal layer comprises metal whichprovides visible spectrum reflectivity to increases the amount of lightextracted through said cathode.
 8. The device of claim 1, wherein theanode modification layer and/or the barrier layer strongly absorbvisible light.
 9. The device of claim 1, wherein the metal layercomprises Al, Cu, Mo, Ti, Pt, Ir, Ni, Au, or Ag, or any alloy thereof,or any metal stack such as Pt on Al.
 10. The device of claim 1, whereinthe metal layer is formed on a substrate.
 11. The device of claim 10,wherein the substrate is a silicon substrate.
 12. The device of claim11, wherein the silicon substrate is crystalline.
 13. The device ofclaim 10, wherein the substrate comprises polysilicon circuitry oramorphous circuitry.
 14. The device of claim 11, wherein the siliconsubstrate comprises integrated circuitry.
 15. The device of claim 1,wherein the anode modification layer is selected to provide efficienthole injection into the organic region.
 16. The device of claim 1,wherein the anode modification layer is selected to form a stableinterface with the organic region.
 17. The device of claim 1, whereinthe anode modification layer is conductive or insulating.
 18. The deviceof claim 1, wherein an oxide layer serves as anode modification layer.19. The device of claim 1, wherein the barrier layer physically andchemically isolates the anode modification layer from the metal layer byforming a barrier, while permitting charge to pass through itsinterfaces with the metal layer and anode modification layer.
 20. Thedevice of claim 1, wherein the barrier layer is conductive orinsulating.
 21. The device of claim 1, wherein the barrier layer ishighly reflective.
 22. The device of claim 1, wherein the anodemodification layer comprises ITO, ZnO, MgO, Sn₂O₃, In₂O₃, RuO₂, or V₂O₅.23. The device of claim 1, wherein the barrier layer comprises a Nilayer and a NiO, layer, and the anode modification layer comprises V₂O₅.24. Display or array comprising at least one device according to claim 1said device being formed on a substrate.
 25. Display of claim 24,wherein said substrate is a silicon substrate.
 26. Display of claim 25,wherein circuitry is integrated into the substrate.
 27. Display of claim26, wherein the circuitry is designed to drive the device.
 28. Displayof claim 24, wherein said anode is patterned.
 29. Method for making anorganic electroluminescent device comprising an anode, a cathode, and anorganic region being sandwiched in between, said method comprising thesteps: building up said anode prior to the formation of said organicregion and cathode, by forming a metal layer, a barrier layer situatedon the metal layer, and an anode modification layer situated on thebarrier layer, said anode being arranged such that said anodemodification layer is in contact with said organic stack and light isemitted through said cathode.
 30. The method of claim 29, wherein anoxide serves as said anode modification layer, said oxide being formedby oxidizing the surface of the barrier layer.
 31. The method of claim29, wherein the anode modification layer is formed by depositing it ontothe barrier layer.